This invention generally relates to metal components employed in a high-temperature environment. The invention is also directed to methods for maintaining the integrity of protective coatings for such metal components.
Many types of metals and metal alloys are used in industrial applications. When the application involves demanding operating conditions, specialty metals are often required. As an example, components within gas turbine engines operate in a high-temperature environment. Many of these components are formed from nickel-base and cobalt-base superalloys. Since the components must withstand in-service temperatures in the range of about 1100° C.-1150° C., the superalloys are often protected with thermal barrier coatings (TBC's).
The TBC's are typically formed of temperature-resistant ceramic materials such as yttria-stabilized zirconia. (In many cases, a metallic bond coat is applied between the TBC and the substrate). In the case of a turbine engine, the thermal barrier coatings are applied to various superalloy surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. The TBC's can be applied over the component by various techniques. Non-limiting examples include physical vapor deposition (PVD); plasma spray techniques (e.g., air plasma spray); and high velocity oxy-fuel (HVOF).
The coefficient of thermal expansion (CTE) of a ceramic TBC and a metallic substrate can differ significantly. Thus, the thermal mismatch which is evident at elevated temperatures can result in damage to the TBC, and/or spallation of the coating from the substrate surface. To minimize the problems associated with such a thermal mismatch, the TBC is often provided with vertical channels or cracks (i.e., vertical to the coating surface). For example, a TBC deposited by a PVD process under selected conditions includes a pattern of substantially vertical microcracks. (Such a TBC is often said to have a “columnar microstructure”). The microcracks permit the TBC to expand and contract with the underlying metal, acting as a stress reliever. The resulting TBC can thus exhibit very good integrity during exposure to high temperatures and frequent thermal cycles. Moreover, TBC's deposited by plasma spray techniques such as air plasma spray (APS) can also contain vertical microcracks, although the microstructure is usually somewhat different from that formed by a PVD process. The vertical microcracks in the APS-applied TBC, as well as other porous regions usually formed in the coating by APS, can also serve as an effective stress reliever for thermal mismatches.
However, the integrity of the TBC can still be compromised under many conditions. For example, spallation of the coating can be promoted as a result of contact with various environmental contaminants. In the case of turbine engines used in aircraft (as well as land-based turbines), examples of the contaminants include, sand, dirt, volcanic ash, fly ash, cement, runway dust, substrate impurities, fuel and air sources, oxidation products from engine components, and the like. The environmental contaminants adhere to the surfaces of thermal barrier coated parts. The contaminant compositions may have melting ranges or temperatures at or below the operating temperature of the turbine component. In the case of a gas turbine engine operating at about 1000° C. or higher, the contaminant compositions often comprise calcium-magnesium-aluminum-silicon-oxide (CMAS) materials.
When a CMAS contaminant becomes molten at the operating temperature of the component, it can infiltrate the TBC. For example, the contaminant can migrate into the microcracks and other porous regions of the TBC. After infiltration and cooling, the molten CMAS (or other contaminant) solidifies. The resulting stress build-up within the coating—especially during additional thermal cycles—can result in spallation of the coating material. Thus, the thermal protection provided to the underlying part may be lost or seriously reduced.
Various techniques have been undertaken to address the problem. For example, a sacrificial oxide coating which reacts with the contaminant material can be applied over the TBC. (The sacrificial coating is sometimes referred to as a “mitigation coating”, and is often an alumina or alumina-based material). As described in U.S. Pat. No. 5,773,141 (Hasz et al), the melting temperature and viscosity of the contaminant composition can increase when it reacts with the sacrificial coating. As a result, the contaminant composition does not become molten, and infiltration of the contaminant into the various cracks, openings and pores of the TBC is minimized or eliminated. Therefore, damage to the TBC can be significantly reduced.
Mitigation coatings have been applied by a number of processes, such as sol-gel, air plasma spray, sputtering, and vapor deposition techniques. A popular vapor deposition technique used for this purpose is the metal-organic chemical vapor deposition process, known as “MOCVD”. As described in U.S. Pat. No. 6,926,928 (Ackerman et al), MOCVD is said to be a “non-line-of-sight” process, in which the oxide coating is deposited upon portions of the substrate that are not visible from an external source. Thus, very good coverage and protection of internal regions (e.g., cracks and pores within a TBC) can be attained. Moreover, MOCVD techniques can be carried out at relatively low substrate temperatures, e.g., in the range of about 350-950° C., which is a considerable processing advantage. Oxide coatings can be deposited to a well-defined thickness by MOCVD as well.
While there are certainly many advantages to using MOCVD to apply mitigation coatings, there are some disadvantages as well. For example, MOCVD can be a very expensive process. MOCVD systems often utilize large reactors, and need to contain a considerable number of other components, such as a vacuum system; a gas mixing cabinet, a cooling system, a heating system (e.g., an RF-generator), computer control systems, a scrubber, and a chiller. This type of system is often designed for handling large numbers of substrates, i.e., components which are being coated. Thus, in terms of efficiency and economy, large MOCVD systems may not be well-suited for handling individual substrates, or small numbers of substrates. Moreover, MOCVD techniques sometimes require relatively long process times, which may not always be ideal.
With these considerations in mind, new methods for applying mitigation coatings over TBC's which include various types of open regions or voids would be welcome in the art. The methods should be capable of at least partially filling the voids with coating material which inhibits the movement and deleterious effects of various contaminant compositions. The methods should also be relatively efficient, and adaptable to economically treating individual substrates coated with the TBC's, or small numbers of the substrates.